The present invention concerns a development unit for electric photocopiers and, more specifically, a development unit using a magnetizable one-component toner initially contained in a removable container of cylindrical shape, from which it is fed across a development chamber towards a rotating development sleeve made of a non-magnetic material inside which are contained a number of stationary permanent magnets.
As is known, on the outer surface of the development sleeve the magnetic field generated by the magnets forms a layer of toner known as the magnetic brush, in which the thickness of the layer is limited by the action of a metering blade made of a non-magnetic material which is fixed, and rests elastically in contact with the surface of the development sleeve.
Various types of development units with magnetic brushes of the above type are known, in which the excess toner on the magnetic brush, which is removed by the metering blade, tends to accumulate upstream from the blade itself with respect to the rotation direction of the development drum, so giving rise to a compacting effect of the toner which forms more or less solid lumps. When this happens, the transport of a uniform layer of toner over the surface of the development sleeve to the development position cannot take place evenly, and defects appear in the developed image. At the same time the toner fed from the removable container accumulates with that already present in the development chamber, also under the magnetising action of the stationary permanent magnets inside the development sleeve, so giving rise to an increase in the density and correspondingly a decrease in the fluidity of the toner itself, with the formation of lumps that give rise to defects in the image developed.
In the present state of the art methods are known both for stirring and fragmenting the lumps of toner by means of mechanical devices, and for avoiding the accumulation of toner in the development chamber, by for example the use of a device to control the feed system discontinuously via a sophisticated control system for the degree of filling of the development chamber, such that the toner fluidity is maintained at a correct level; however, such devices have the disadvantages of requiring costly and complex solutions, and of not being completely effective.
Moreover, in present magnetic brush development units, in which the one-component toner is contained and extracted mechanically from a container that can be removed from the development unit, toner compaction can occur inside the removable container, during its transport and storage. When the said removable container is inserted into a development unit, this compaction gives rise to a considerable increase in the magnitude of the torque that needs to be applied to the rotating extraction device to bring the one-component toner out of the removable container, such that in the worst cases the weakest part of the extraction device itself can break.
Still referring to the present state of the art concerning magnetic brush development units in which the one-component toner is contained in a container that can be removed from the development unit, a problem arises indicating that the removable container has become empty and must be replaced by a full one. This entails the two contrasting requirements of guaranteeing that the container is completely empty and that the development chamber still contains sufficient toner to form a uniform layer over the surface of the development drum. In fact, on the one hand incomplete emptying of the removable container of one-component toner, besides incurring additional cost due to wastage of unused toner and making it more difficult to dispose of the removable container in an ecologically acceptable way, can easily give rise to soiling of the electric photocopier, the area around it, and perhaps even the operator who is removing the removable container from the development unit; on the other hand, if the quantity of residual toner in the development chamber is reduced to the point where it is no longer possible to obtain a uniform toner layer over the surface of the development sleeve, this will produce defects in the image developed. Finally, in the present state of the art concerning magnetic brush development units of the type described above, in which the height of the toner layer forming the magnetic brush is limited by the action of a metering blade, it is difficult to devise simple systems to prevent the one-component toner becoming compressed between the outside surface of the development sleeve and the lower face of the metering blade. This gives rise to the formation of a thin film formed of the resin constituting the one-component toner over the surface of the development sleeve. This film alters the mechanical and the triboelectric characteristics of the surface of the development sleeve, making it critical to form a uniform toner layer over the surface of the drum and hence giving rise to defects in the developed image.